In recent years, there has been regarded an influence of polarization mode dispersion (hereinafter, referred to as PMD) as a factor limiting long-distance transmission and speeding up of optical communication. PMD means a phenomenon that two modes having planes of polarization perpendicular to each other propagate through a fiber at a slightly different speed and thus a width of incident pulse enlarges as the incident pulse propagates through the fiber.
A normal single mode optical fiber has a core part and a clad part having a refractive index smaller than that of the core part on the outer circumference, and incident light is approximately confined within the core part and advances in such a state. When a core part is perfectly a complete round, these two modes cannot be distinguished due to degeneracy. However, when a core part is not a complete round or distortion is added to the core part in a step manufacturing an optical fiber or an optical cable, the two modes propagating through the optical fiber have a different speed and thus PMD occurs because a symmetric property is broken.
If an influence of PMD becomes large, signal light pulses different from each other are overlapped during optical communication and thus sometimes the detection of signal light pulse becomes impossible. Therefore, it becomes important to closely manage non-circularity of an optical fiber core part when a PMD characteristic is strictly requested particularly.
Non-circularity of a core part of an optical fiber can be measured by means of, e.g., a near field image method. However, since a diameter of a core part of a single mode optical fiber is extremely small in the order of 10 μm to the utmost, this lacks for measurement accuracy and the measurement of non-circularity is easy to be influenced by a state of a measuring plane when the optical fiber has been cut. Therefore, it is desirable that non-circularity of a core part is measured in a step for a preform for optical fiber (hereinafter, simply referred to as a preform).
Non-circularity of a preform can be obtained, e.g., by measuring an outside diameter of the preform from a plurality of directions by means of a laser beam diameter measurement device while rotating the preform and dividing a difference between a maximum value and a minimum value of the obtained outside diameter data by a mean value. In addition, it is possible to calculate an expression of an ellipse by means of a least-squares method or the like and obtain non-circularity from length of a major axis and a minor axis and a mean value therefor.
However, non-circularity of a core part of a preform cannot be measured by the above method because the core part is usually synthesized integrally with a part of a clad part.
Patent Document 1 proposes a method of obtaining non-circularity of a core part based on light intensity distribution for image capturing obtained by horizontally arranging an optical fiber base material 2 so that the base material penetrates a vessel 1 filled with liquid having a refractive index substantially equal to that of a clad part, irradiating parallel light from a light source section 3 toward a side face of the optical fiber base material 2, and receiving the transmitted light by a light receiving section 4, as shown in FIG. 1. This method is to measure a width of a bright space of light intensity distribution to obtain a relative value for a core diameter and obtain non-circularity of a core part from this relative value.
[Patent Document 1] Japanese Patent Application Publication No. 2003-42894